The present invention is in the field of optical communications, especially in free-space optics.
Data services and capacity requirements have been steadily growing in the past. High data rate services become more and more accepted and appreciated. During the past decade, the backbone network experienced enormous growth in capacity and reliability, mainly due to major development efforts in the area of optical networking. During the same time, the bandwidth demands of technology-savvy end users for broadband services such as third-play (voice, video and Internet) have also increased at an unprecedented rate. The access network still remains a bottleneck for providing bandwidth-intensive services to customers. Technologies such as digital subscriber line (DSL) and cable modem (CM) cannot carry the high volume of traffic generated by emerging applications such as video-on-demand (VoD), interactive gaming, or duplex videoconferencing. Thus, future access technologies will provide high capacity and operational efficiencies along with mobility support access to users in a cost-effective manner, P. Chowdhury, B. Mukherjee, D. Sarkar, G. Kramer and S. Dixit, “Hybrid Wireless-Optical Broadband Access Network (WOBAN): Prototype Development and Research Challenges,” IEEE Network Magazine, Vol. 23, No. 3, May/June 2009, pp. 41-48.
Optical fiber-based technologies are well suited to support integrated high-bandwidth digital services and can alleviate bandwidth bottlenecks, but laying fiber infrastructure to all end users incurs significant cost. Wireless access networks on the other hand, necessitate less infrastructure deployment, cf. S. Ou, K. Yang, M. P. Farrera, C. Okonkwo, K. M. Guild: A control bridge to automate the convergence of passive optical networks and IEEE 802.16 (WiMAX) wireless networks, Proceedings of the Fifth IEEE Broadband Communications, Networks and Systems Conference, September 2008. Furthermore, users also desire untethered access, especially if they are mobile. Wireless technologies can support mobility and untethered access. The integration architectures can take advantage of the bandwidth benefit of fiber communications, and the mobile and non-line-of-sight features of wireless communication, compare G. Shen, R. S. Tucker, C.-J. Chae: Fixed mobile convergence architectures for broadband access: Integration of EPON and WiMAX, IEEE Communications Magazine, Vol. 45 (2007) No. 8, pp. 44-50.
Optical and wireless technologies are expected to coexist over the next decades, cf. N. Ghazisaidi, M. Maier and C. Assi, “Fiber-wireless (FiWi) access networks: a survey,” IEEE Communications Magazine, Vol. 47 (2009) No. 2, pp. 160-167. Also, it is anticipated that radio communications will be complemented by wireless optical communications. In particular, the deployment of Free-Space Optical (FSO) communication systems is considered a viable approach.
In the following, some basics on optical communications will be introduced. In general, i.e. for radio as well as for optical communications, at a transmitter, a complex baseband signal s(t) is converted into a transmission band or a bandpass signal. FIG. 4a illustrates this conversion. The baseband signal is converted from a complex domain into a propagating wave, having a carrier frequency ω and a possibly retarded wave propagation through free-space according to a wave vector {right arrow over (k)}. The transmitter converts the signal into a wave, propagating along the wave vector {right arrow over (k)}. FIG. 4b defines the wave vector mathematically. In FIG. 4b, λ corresponds to the wave length of the respective wave. In other words, the complex baseband signal s(t) is converted from a single dimension to four dimensions, namely the frequency dimension plus three spatial dimensions.
It is to be noted that there is a significant difference between radio communications and optical communications with respect to the wavelengths. While in radio communications the wavelength ranges approximately between 10 cm to 40 cm, the wavelength for optical communication ranges between 40 to 70×10−6 cm. In other words, the wavelength in radio communications is about twice the length of an antenna and, therefore, each wave half period can be detected independently of the others. This can enable coherent detection. In optical communications, the wavelength is about five orders of magnitude smaller than the size of a photo-detector, which averages over many wave periods and, therefore, the signal fades. Coherent detection is not possible and photo-detectors detect the intensity only.
FIG. 4c provides a view chart, illustrating a signal s(t) over time. The signal shown in FIG. 4c has positive and negative values, which is not possible for optical communications. For optical communications, only the positive parts of s(t) are possible, the negative parts are forbidden. Light sources are physically limited, since the intensity cannot be negative. Therefore, an optical signal is bounded as illustrated by FIG. 4d showing a similar view chart as FIG. 4c, however, this time, the signal s(t) only shows positive values. Thus, the baseband signals in free-space optics can only be On-Off Keying (OOK) modulated. On-Off Keying modulation is a rather basic modulation method where a signal is switched on and off as indicated in FIGS. 4c and 4d. 
FSO systems like Infrared (IR) remote controls deploy Intensity Modulation (IM), transmitting light from an incoherent light source, typically an LED, or transmitting coherent light using e.g. a laser diode. The transmitted light is switched on and off based on a unique predetermined information sequence, yielding On-Off Keying (OOK), cf. e.g. M. Huchard, M. Weiss, Anna Pizzinat, S. Meyer, P. Guignard, B. Charbonnier: Ultra-broadband wireless home network based on 60-Ghz WPAN cells interconnected via RoF: IEEE Lightwave Journal, Vol. 26 (2008) No. 15, pp. 2364-2372 and the references therein.
Future wireless systems and mobile communication systems are foreseen to utilize optical transmission components. Some of the systems may utilize FSO. Another optical transmission technology uses guided waves as, for example, fiber optics. Some FSO systems as, for example, used in remote controls, use intensity modulation (IM), which switches on and off the emitted light of an incoherent light source as, for example, an LED or even of a coherent light source as, for example, a laser diode.
OOK is rather simple to implement, however, provides a disadvantage of a low spectral efficiency, i.e. a low transmission rate per bandwidth. Conventional concepts may utilize quadrature modulation on the transmitter side and direct mixing on the receiver side, where a receiver uses a coherent light source as a laser as well, cf. W. Shieh, C. Athaudage: Coherent optical orthogonal frequency division multiplexing, IET Electronics Letters, Vol. 42, No. 10, S. 587-588, 2006 and W. Shieh, H. Bao, Y. Tang: Coherent optical OFDM: Theory and design, Optics Express, Vol. 16 (2008) No. 2, pp. 841-859 and A. K. Anandarajah, P. Perry, L. P. Barry: Hybrid radio over fiber system for generation and distribution of UWB signals, Proceedings of the Tenth IEEE ICTON, Vol. 4, (June 2008), pp. 82-85. Other conventional concepts use incoherent envelope detection, however, assume a real valued modulation, cf. H. Paul, K.-D. Kammeyer: Modeling and influences of transmitter and receiver nonlinearities in optical OFDM transmission, Proceedings of the 13th International OFDM Workshop 2008 (InOWo '08), Hamburg, August 2008. Conventional concepts generally favor single-side band modulation (SSB), because of its simpler implementation on the receiver side.
On the transmitter side, conventional concepts utilize laser diodes and Mach-Zehnder interferometers for optical signal creation, cf. W. Shieh, H. Bao, Y. Tang: Coherent Optical OFDM: Theory and Design, Optics Express, Bd. 16, Nr. 2, S. 841-859, January 2008; M. Mayrock, J. Haunstein: Impact of Implementation Impairments on the Performance of an Optical OFDM Transmission System, Proceedings of 32nd European Conference on Optical Communications (ECOC), Cannes, France, September 2006 and A. Ali, J. Leibrich, W. Rosenkranz: Spectral Efficiency and Receiver Sensitivity in Direct Detection Optical-OFDM, OFC 2009, paper OMT7, San Diego, Calif., USA, 22-26.03.2009, On the receiver side, another laser diode is used with a number of photo detectors, cf., for example, FIG. 6 of W. Shieh, H. Bao, Y. Tang: Coherent Optical OFDM: Theory and Design, Optics Express, Bd. 16, Nr. 2, S. 841-859, January 2008.
OOK is spectrally inefficient, allowing only small data rates at a given bandwidth. This disadvantage can be overcome by using Higher Order Modulation (HOM), e.g. combined with multicarrier transmission schemes like Orthogonal Frequency Division Multiplexing (OFDM). To implement such optical transmission systems based on HOM and OFDM, often, quadrature modulation is used at the transmitters and direct mixing concepts are used at the receivers, e.g. based on lasers or LDs, cf. e.g. W. Shieh, C. Athaudage: Coherent optical orthogonal frequency division multiplexing, IET Electronics Letters, Vol. 42, No. 10, S. 587-588, 2006. Other receiver concepts are based on incoherent envelope detection schemes, assuming real valued modulation schemes, cf. H. Paul, K.-D. Kammeyer: Modeling and influences of transmitter and receiver nonlinearities in optical OFDM transmission, Proceedings of the 13th International OFDM Workshop 2008 (InOWo '08), Hamburg, August 2008. OFDM has also been treated in e.g. A. M. J. Koonen, M. G. Larrod´e, A. Ng'oma, K. Wang, H. Yang, Y. Zheng, E. Tangdiongga: Perspectives of Radio over Fiber Technologies, Proceedings of the OFC/NFOEC, (February 2008), pp. 1-3. Subcarrier Modulation (SCM) was discussed in K. Kazaura, K. Wakamori, M. Matsumoto, T. Higashino, K. Tsukamoto, S. Komaki: A proposal for a broadband wireless access technology based on radio-on-FSO links, Proceedings of the IEEE GLOBECOM 2008, (November/December 2008), pp. 1-6.
The optical signal generation at the transmitter is usually based on LDs combined with at least one external Mach-Zehnder modulator, cf. D. Wake, K. Beacham: A novel switched radio over fiber architecture for distributed antenna system. Proceedings of the 17th IEEE LEOS, Volume 1 (November 2004), pp. 55-56, D. Wake, M. Webster, G. Wimpenny, K. Beacham, L. Crawford: Radio over fiber for mobile communications, Proceedings of the IEEE MWP 2004, (October 2004), pp. 157-160, M. Mayrock, H. Haunstein: Impact of implementation impairments on the performance of an optical OFDM transmission system, Proceedings of 32nd European Conference on Optical Communications (ECOC), Cannes, France, September 2006, A. K. Anandarajah, P. Perry, L. P. Barry: Hybrid radio over fiber system for generation and distribution of UWB signals, Proceedings of the Tenth IEEE ICTON, Vol. 4, (June 2008), pp. 82-85, M. Arief, M. Sevia, M. Idrus, S. Alifah: The SCM/WDM system model for radio over fiber communication link, Proceedings of the IEEE RFM 2008, (December 2008), pp. 344-347, W. Shieh, H. Bao, Y. Tang: Coherent optical OFDM: Theory and design, Optics Express, Vol. 16 (2008) No. 2, pp. 841-859, A. Ali, J. Leibrich, W. Rosenkranz: Spectral efficiency and receiver sensitivity in direct detection optical-OFDM, OFC 2009, paper OMT7, San Diego, Calif., USA, March 2009, M. Hossen, B.-J. Jang, K.-D. Kim, Y. Park: Extension of wireless sensor network by employing RoF based 4G network, Proceedings of the Eleventh ICACT 2009, (February 2009), pp. 275-278; M. Morant, T. F. Alves, R. Llorente, A. V. T. Cartaxo, J. Marti: Experimental comparison of transmission performance of multichannel OFDM-UWB signals on FTTH networks, IEEE Lightwave Journal, Vol. 27 (2009), No. 10, pp. 1408-1414 and M. T. Riaz, R. H. Nielsen, Pedersen, J. N. Prasad, O. B. Madsen: On radio over fiber for heterogeneous wireless networks, Proceedings of the Ninth IFIP WOCN, (April 2009), pp. 1-4.
At the receiver, LDs and often several Photo Detectors (PDs) are used, cf. e.g. FIG. 6 of W. Shieh, H. Bao, Y. Tang: Coherent optical OFDM: Theory and design, Optics Express, Vol. 16 (2008) No. 2, pp. 841-859.
Direct modulation at the transmitter side has been presented in S. Sabesan, M. Crisp, R. V. Penty, I. H. White: Demonstration of improved passive UHF RFID coverage using optically-fed distributed multi-antenna system, Proceedings of the IEEE RFID 2009, (April 2009), pp. 217-224 and H. Yeh, C. W. Chow, F. Y. Shih, C. H. Wang, Y. F. Wu, Y. LiuI, D. Z. HsuI, Allan LinI, Denial Mai, S. Chi: Performance and limitation of radio-over-fiber network using standard WiMAX signal, Proceedings of the IFIP WOCN 2009, (April 2009), pp. 1-4.
Further aspects, are resource management and quality of service, as e.g. treated in the FUTON project, cf. D. Wake et al, H. B. Kim, M. Emmelmann, B. Rathke, A. Wolisz: A radio over fiber network architecture for road vehicle communication systems, Proceedings of the 61st. IEEE Vehicular Technology Conference, Vol. 5 (June 2005), pp. 2920-2924; S. R. Chaudhry, H. S. AL-Raweshidy: Application-controlled handover for heterogeneous multiple radios over fibre networks, IET Communications, Vol. 2 (2008) No. 10, pp. 1239-1250 and M. Kamoun, S. Yang, M. D. Courville: Multi-RAU pilots for ROF enabled distributed antenna systems, Proceedings of the First Wireless VITAE (May 2009), pp. 177-181.
Another aspect are remote antennas with fibre connections connections, cf. L. Chen, J. G. Yu, S. Wen, J. Lu, Z. Dong, M. Huang, G. K. Chang: A novel scheme for seamless integration of RoF with centralized light wave OFDM-WDM-PON system, IEEE Lightwave Journal, Vol. 27 (2009) No. 14, pp. 2786-2791 and S. Sabesan, M. Crisp, R. V. Penty, I. H. White: Demonstration of improved passive UHF RFID coverage using optically-fed distributed multi-antenna system, Proceedings of the IEEE RFID 2009, (April 2009), pp. 217-224.
Background information about RoF (Radio over Fiber) with focussed beams can be found in C. Santiago, B. Gangopadhyay, A. M. Arsenio, M. V. Ramkumar, N. R. Prasad: Next generation radio over fiber network management for a distributed antenna system, Proceedings of the First IEEE Wireless VITAE, (May 2009), pp. 182-186.
Conventional methods on UWB (Ultra Wide Band) over fibre using laser-based Mach-Zehnder modulators can be found in Z. Jia, J. Yu, G.-K. Chang: A full-duplex radio-over-fiber system based on optical carrier suppression and reuse, IEEE Photonics Tech. Lett., vol. 18 (2006) No. 16, pp. 1726-1728; J. Tang, X. Jin, Y. Zhang, X. Zhang, W. Cai: A hybrid radio over fiber wireless sensor network architecture, Proceedings of the WiCOM 2007, (September 2007), pp. 2675-2678 and S. Kuwano, Y. Suzuki, Y. Yamada, Y. Fujino, T. Fujita, D. Uchida, K. Watanabe: Diversity technique employing digitized radio over fiber technology for wide-area ubiquitous network, Proceedings of the IEEE GLOBECOM 2008, (November/December 2008), pp. 1-5.
Switching aspects are illuminated, for example, in I. Gasulla, J. Capmany: Simultaneous baseband and radio over fiber signal transmission over a 5 km MMF link, Proceedings of the IEEE MWP/APMP 2008, (September/October 2008), pp. 209-212 and S. Sabesan, M. Crisp, R. V. Penty, I. H. White: Demonstration of improved passive UHF RFID coverage using optically-fed distributed multi-antenna system, Proceedings of the IEEE RFID 2009, (April 2009), pp. 217-224. The backbone architecture is considered by A. Osseiran, E. Hardouin and A. Gouraud, M. Boldi, I. Cosovic, K. Gosse, J. Luo, S. Redana, W. Mohr, J. F. Monserrat, T. Svensson, A. Tölli, A. Mihovska, M. Werner: The road to IMT-advanced communication systems: State-of-the-art and innovation areas addressed by the WINNER+ project, IEEE Communications Magazine, (June 2009), pp. 38-47.